CT / PT Analyzers
Knee point, ratio, burden and resistance from one set of leads
A protection current transformer (CT) that saturates before the relay operates is a protection scheme that does not exist. The knee point tells you where that happens, and it is not something the nameplate can be trusted for after twenty years in service. A CT/PT analyzer measures the excitation characteristic, ratio, polarity, burden and winding resistance on one instrument, in the substation, with the transformer still in place.
Atlas Energy's analyzer drives CT excitation to 2,500 V (3,000 V optional) at up to 20 A, computes the knee point automatically, and derives the 5%, 10% and 15% error curves at 1%, 5%, 20%, 100% and 120% of rated. Ratio error is held to ±0.5% and phase error to ±3 minutes. Primary injection to 200 A for up to five minutes verifies the metering circuit including its wiring. Results land on a 7-inch color touchscreen with a built-in printer, and 4,000 records export by USB for PC reporting. The unit weighs 26 kg, measures 420 × 300 × 270 mm and works from −10 °C to +50 °C.
Why it matters
The knee point is computed automatically from the measured excitation curve, so protection CT saturation becomes a measured value instead of a nameplate assumption.
Ratio error to ±0.5% and phase error to ±3 minutes support revenue metering acceptance against IEEE C57.13 and IEC 61869-2.
Primary injection to 200 A for up to five minutes tests the transformer and its secondary wiring together, which is where commissioning errors actually live.
Nine measurements come from one set of leads: excitation, error curves, ratio and polarity, phase error, secondary resistance, burden, continuity, secondary withstand and demagnetization.
Demagnetization is built in, so the CT is left in a defined state after DC-based tests rather than carrying remanence into service.
Ratings
Supply: 220 VAC ±10%, 50 Hz ±10%, maximum 5 kVA
CT excitation output: 0–2,500 V (3,000 V optional), 20 A maximum, ±0.5%
CT ratio test current: 0–600 A (1,000 A optional), ±0.5%
PT excitation: 0–500 V, 20 A maximum, ±0.5%
PT ratio: 0–2,500 V (3,000 V optional), ±0.5%
Ratio error: ±0.5%; phase error ±3 minutes
Secondary DC resistance: 0–20 Ω, 0.5% accuracy
Primary injection: 0–200 A for up to 5 minutes
Secondary withstand: 0–2,500 V for up to 5 minutes
Display: 7-inch color touchscreen with built-in printer
Memory: 4,000 test records with USB export and PC report generation
Weight and dimensions: 26 kg; 420 × 300 × 270 mm; −10 °C to +50 °C
Extended range
Excitation characteristic: Volt-ampere curve with automatic knee-point calculation
Error curves: 5% / 10% / 15% computed at 1%, 5%, 20%, 100% and 120% of rated
Turns ratio and polarity: Measured directly
Ratio and phase error: ±0.5% ratio, ±3 minutes phase
Secondary winding DC resistance: 0–20 Ω at 0.5% accuracy
Secondary burden: Measured on the connected circuit
Secondary circuit continuity: Verified before and after testing
Secondary withstand voltage: 0–2,500 V for up to 5 minutes
Demagnetization: Built-in routine after DC-based tests
Standards
IEEE C57.13
IEC 61869-2 (CT)
IEC 61869-3 (VT)
Applications
Instrument transformer acceptance and commissioning
Revenue metering accuracy verification
Protection CT saturation and knee-point verification
Substation maintenance up to 500 kV class
Questions engineers ask
Can the analyzer find the knee point of a protection CT? Yes. It drives the excitation output to 2,500 V (3,000 V optional) at up to 20 A, records the volt-ampere characteristic and calculates the knee point automatically. The 5%, 10% and 15% error curves are computed at 1%, 5%, 20%, 100% and 120% of rated, which is the form IEEE C57.13 and IEC 61869-2 assessments are made against.
Does the same instrument test voltage transformers? Yes. PT excitation runs 0–500 V at up to 20 A and PT ratio runs to 2,500 V (3,000 V optional), both to ±0.5%, with phase error to ±3 minutes. IEC 61869-3 is the governing standard on the VT side. Secondary burden, winding resistance and circuit continuity are measured from the same connection.
